The present invention relates to a draw-back valve assembly for an injection molding apparatus. More particularly, the present invention concerns a draw-back valve assembly for an injection molding apparatus wherein flow through the valve assembly is controlled. The present invention is also directed to a draw-back valve assembly for an injection molding apparatus wherein pressure spikes are eliminated.
In injection molding machines a helical screw in the plasticizer barrel is used as a ram to force thermoplastic material ahead of the screw into the mold. When the screw moves forward to its maximum injection position, it is necessary to maintain it in this position for a period of time which is called the "cure cycle". During the cure cycle, the screw maintains a pressure on the material to compensate for shrinkage of material which cools in the mold. Some additional material is forced into the mold to compensate for this shrinkage.
The mold area is cooled so that the part being molded becomes solidified. After the cure cycle has been completed, the reciprocating screw plasticizer begins to rotate and forces plastic toward the discharge end of the barrel. The screw continues to rotate until a predetermined amount of material is accumulated at the front of the screw for the next injection operation. The amount of material accumulated is controlled by a limit switch which stops the screw and moves the non-rotating screw back so as to create a suction in the passages leading to the mold. This draws the hot runner material away from the mold and the mold is then opened and the molded part removed.
The objection to the aforementioned method of operation is that the molded part cannot be removed from the mold until the next injection shot has been plasticized and accumulated ahead of the plasticizer screw. U.S. Pat. No. Re. 28,721 to the applicant herein, the entire contents of which are incorporated by reference herein, served to reduce the auxiliary of operation of an injection molding machine by making it possible to remove the molded article much sooner. In U.S. Pat. No. Re. 28,721, no drawback of the plasticizer screw is required and the preparation of the next injection charge can be prepared starting immediate after preparatory injection of the previous charge and without waiting for any part of the cure cycle to elapse.
In U.S. Pat. No. Re. 28,721, there was provided a valve between the mold and the discharge end of the plasticizer valve. An auxiliary cylinder and piston was used to maintain pressure on the material in the mold during the cure cycle and the piston in this auxiliary cylinder was reversed in direction to draw back the runner material prior to discharge of the molded part from the mold. Thus, U.S. Pat. No. Re. 28,721 made it possible to open the mold, remove the molded part, and close the mold preparator to the next operation while the plasticizer screw was plasticizing the next quantity of material for the molding of another part. U.S. Pat. No. Re. 28,721 served to greatly reduce the overall cycle of the apparatus.
Although U.S. Pat. No. Re. 28,721 represented a remarkable advancement in the field of injection molding, there remains a need to control the throughput of extruded material through the draw-back valve of U.S. Pat. No. Re. 28,721 so as to restrict flow to more slowly fill the mold. Furthermore, there remains a need to eliminate the pressure spikes inherent in the use of the draw-back valve assembly of U.S. Pat. No. Re. 28,721.